The sweet receptor, composed of the monomers T1R2+T1R3, is the principal receptor responsible for sweet taste in mammals. T1R2 and T1R3 are two related G protein-coupled receptors of the T1R type I taste receptor family. Together, T1R2+T1R3 respond to all classes of sweet compounds. TIRs have a large extracellular domain consisting of a venus flytrap module and a cysteine-rich domain; a 7-transmembranes domain; and a short C-terminal tail. The primary goal of this application is to elucidate structure-function relationships underlyinghuman T1R2+T1R3 dimeric receptor interactions with the sweet tasting protein monellin. I will take advantage of the fact that monellinacts on the human sweet receptor, but not on the mouse receptor to identify the specific residues responsible for the human receptor's response tomonellin. Our previous work with human/mouse chimeras has shown that the amino-terminal domain (ATD) of human T1R2 (hT!R2) is critical for the response to monellin. We also have shown that the cysteine-rich domain (CRD) of T1R3 plays a role in monellin's activation of the receptor. The first aim of this proposal is to identify residues within the ATD of T1R2 critical for sweet receptor sensitivity to monellin. To accomplish this aim 1will use chimeras of T1R2 containing varyingparts of human and mouse T1R2, and carry out site- directed mutagenesis. The second aim is to identify residues within the CRD of hT!R3 critical for sweet receptor sensitivity to monellin. This aim will be accomplished by directed mutagenesis of the CRD based on our initial identification of residues in this region of the receptor that are critical for responses to monellin. Knowledge of the structure/function relationship of TIRs may be valuable in the development of novel, low caloric sweeteners to help fight against diet-related disorders such as obesity and diabetes.